1. Field of the Invention
The present invention relates to optical equipment such as a video camera, digital camera, and so forth.
2. Description of the Related Art
With regard to cameras with non-interchangeable lenses, there are demands for reduction in size, the ability to photograph subjects from as close as possible, and so forth. Therefore, rather than interlocking a correcting lens and a variable power lens mechanically by a cam, a so-called inner focus type lens is becoming mainstream, wherein the movement locus of the correcting lens is input in advance as lens cam data within a microcomputer, which drives the correcting lens according to this lens cam data, and further sets the focus by means of this correcting lens.
FIG. 8 is a figure that illustrates the configuration of a lens system with the current inner focus type. Here, reference numeral 901 denotes a fixed front lens, 902 denotes a zoom lens for variable power (also referred to as a variator lens: first lens unit), 903 denotes an aperture diaphragm, 904 denotes a fixed lens that is fixed, and 905 denotes a focus lens (second lens unit) to be used as a correcting lens, which provides a focal point adjusting function and a function that corrects the movement of the image based on the variable power. Further, 906 denotes an imaging plane.
With a lens system configured as in FIG. 8, the focus lens 905 provides both a compensator function and a focal point adjusting function, therefore even if the focal point distance is equal, the position of the focus lens 905 for converging with the imaging plane 906 differs based on the subject distance. When the subject distance is changed with regard to each focal point distance, and when the position of the focus lens 905 is continuously plotted to focus with the subject image on the imaging plane 906, the result is as shown in FIG. 9. While zooming in or out, selecting a locus corresponding to the subject distance from the multiple loci illustrated in FIG. 9 moves the focus lens 905 according to the selected locus, thereby enabling variable power (zooming) while storing the focused state.
Now, with regards to the type of a lens system where the front lens performs the focusing, a focus lens independent of the zoom lens is provided, and further, the zoom lens and the focus lens are mechanically linked to a cam ring. Therefore, in the case of trying to rotate the cam ring manually and change the focal point for example, even if the cam ring is moved very fast, the cam ring follows and rotates. The zoom lens and the focus lens move in the direction of the optical axis, according to the cam formed by the cam ring, and therefore, is the focus lens is in a converging position, the image will not blur due to zooming.
In contrast to this, a lens system of an inner focus type generally records in memory the information of the multiple loci illustrated in FIG. 9 (also called electronic cam locus) or the information corresponding to this (in other words, either information indicating the locus itself, or a function wherein the lens position is a variable, is suitable), selects a locus based on the positions of the focus lens and the zoom lens, and performs zooming while moving along the selected locus.
Now, in the case that the zoom lens moves in the direction from telephoto to wide angle, focus can be maintained using the above-described locus following method, because it converges from a state wherein multiple loci have a given amount of spacing in between, as is apparent from FIG. 9. However, in the direction from wide angle to telephoto, the focus lens that was at the convergence point is uncertain which locus to follow, and therefore focus cannot be maintained with a similar locus following method.
Therefore, Japanese Patent No. 2,795,439 (Claims, FIGS. 3 and 4, and the description thereof) discloses a control method (zigzag movement) wherein, using an AF evaluation value signal (sharpness signal) obtained from the high frequency component of the image signal by using a TV-AF method, when moving the zoom lens (variable power), the focus lens is forced to move so as to be off focus from the focus position, and further, performs repeated control of switching and moving the focus lens toward the direction of focus (changing the following speed to the locus), thereby correcting the following locus. Further, Japanese Patent No. 2,795,439 discloses a method of changing the increase/decrease cycle of the sharpness signal by means of changing the amount of change of the following speed corresponding to the subject, the focal point distance, and the depth of field, and attempts improvement of selection (generating) accuracy of the following locus selection.
The zigzag movement disclosed in the aforementioned Japanese Patent No. 2,795,439 specifies the following locus based on the change of the AF evaluation value. Further, the evaluation value changes not only according to the status of blurring of the image, but also changes according to the pattern changes of the subject. Therefore, considering that there may be cases wherein the focus lens movement switching is switched in the wrong direction, the correction range of the following locus is set as a wide range so as to be able to return to the correct locus even if initially moving in the wrong direction.
On the other hand, when the setting is for such a wide correction range, in the event that the movement deviates from the locus that should be used, the image may blur until moved back to the correct locus. Further, in the case of moving the focus lens in the wrong direction, image blurring may occur wherein the AF evaluation value level in particular is greatly decreased, or when image-taking a subject with low contrast, the correct locus may not be found, and there is the possibility that the image blur is carried all the way to the telephoto edge.
Further, in the case of image-taking a subject with a high frequency, when the following locus is trying to be set by means of the zigzag movement, arbitrary image blurring can occur. In order to make this type of image blurring less conspicuous, the AF evaluation value level, which determines the reverse timing of the drive direction of the focus lens wherein the zigzag movement can be adjusted according to the subject conditions, can be adjusted, but eliminating the occurrence of all image blurring of the subject related to the zigzag movement is difficult.
Further, with the TV-AF method, due to the signal detection cycle obtained by the AF evaluation value being a vertical synchronizing signal cycle, the sharpness of locus selection becomes poorer as the zooming speed becomes faster, and consequently, the rate of mistaken following locus selection increases.